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CCP5 Annual Meeting 2010Invited lecturers at the conference will include: Fernando Escobedo (Cornell) Mike Allen (Warwick) "Computer Simulation of Soft and Biological Matter" I shall describe some recent simulations, using simple models, of peptides in biological membranes, and peptide adsorption at surfaces. In both cases, advanced sampling techniques are helpful in accessing states of the system that would be difficult to observe otherwise. Jamshed Anwar (Bradford) "Free Energies of Nanocrystals from Molecular Simulation" Nanocrystals and their composites can exhibit markedly different properties with respect to bulk phases and hence offer new opportunities. For pharmaceuticals, nanocrystals promise to resolve the issue of poor bioavailability of poorly soluble drugs. The immense surface area enables these crystals to dissolve much quicker, resulting in higher bioavailability. The difficulty in exploiting this technology, particularly for organics, is the technical challenge of generating and stabilising nanocrystals, which is severely limited by the lack of a theoretical framework. Nanocrystal stability is also of interest for fundamental reasons as structural phase transformations in nanocrystals can serve as proxies for investigating the earliest stages of crystallization, more specifically the solid-solid or solvent-induced transformations that characterise the post-nucleation stages of Ostwald’s rule. Going down the length scales gives rise to some new physics. As a crystal decreases in size, its surface or interfacial free energy (the choice depends on whether the crystal is stand alone or surrounded by some medium) becomes significant relative to its bulk free energy and the thermodynamics of the crystal are now determined by the interplay of these two energies. Depending on the crystal size, the surface (interfacial) energy can favour a particular phase that otherwise may be unstable in the bulk or even an entirely new phase, hence enabling phase stability to be modulated. A current difficulty is that it is generally not possible to reconcile this theoretical framework with experimental observations for a given system as the key thermodynamic quantities, surface and bulk free energies, are not readily accessible. Furthermore, there are other fundamental issues: how does the phase diagram for a system exhibiting multiples phases vary as we go down the length scale? At what length scale do the macroscopic ideas of bulk free energy (periodic structure) and surface (interfacial) free energy break down? How are the kinetic barriers to phase transformation influenced by length scale? I shall present some preliminary calculations of free energies of bulk surfaces and of nanocrystals of the Lennard Jone’s model using thermodynamic integration. These represent precursor studies towards addressing some of the fundamental issues raised above. Pep Espanol (Madrid) "Coarse-Graining Fluids and Big Molecules" There is a large interest in multiscale simulations that should allow one to reach larger samples and time scales with available computer resources. Understanding the link between different levels of descriptions is crucial to this venture. In this talk, I will show how statistical mechanics tackles this problem of coarse-graining in the case of bonded and non-bonded interactions, and will illustrate the general theory in a couple of examples. I will also point to some of the open questions that one has to face in order to reach the objectives of coarse-graining. Erich Muller (Imperial) "Coarse-Grained Simulation of the Self-Assembly and Mesophase Behaviour of Polyphilic Liquid Crystals" We present a series of molecular dynamics studies into the thermotropic liquid crystalline phase behaviour of three distinct families of macromolecular liquid crystals. In all cases, the molecules are aggressively coarse-grained, using as building blocks soft repulsive or attractive spheres. The first family of molecules studied are rigid bolaamphiphilic molecules with grafted side chains. The nature of the side chains is antagonistic with the core of the molecule and this polyphilicity accounts for a very rich mesophase behaviour. Up to 11 different mesophases are encountered in the pure fluid alone. The mixture behavior with explicit coarse grained solvents is also studied and shows some unexpected phase behaviour. A second family of molecules consists of shape persistent macrocycles, where a very large hoop-like molecule is decorated with intra-annular and extra-annular attractive side chains. In this case, depending on the characteristic of the side chain, a unique smectic phase with anti-nematic ordering is seen or a more common columnar liquid crystalline phase is detected. A third family of systems consists of dendritic molecules. For these type of models, we are able to reproduce the recent experimentally observed pseudo-crystal phases, consisting of a crystal-like arrangement of self-assembled mono-sized macropheres. The simulations shed light on these phases and relate them to other apparently unknown phases, corresponding to a liquid-like version and a columnar version of the parent pseudo crystal phase. The DL_POLY suite is used in all these simulations, and the talk focuses on the capability of these coarse-grained methods along with parallel MD programs to map out the global phase behaviour of very complex macromolecules.
Last modified 04 June 2010 |
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